This invention relates generally to gas turbine combined cycle systems, and more particularly to systems having a gas turbine and a heat recovery steam generator.
In the power generation industry, various systems are known in which gas turbines, steam generators and steam turbines are combined for reasons of efficiency. One such system employs a gas turbine in conjunction with a steam generator in which the gas turbine is co-generative, as it not only provides power, but also produces a hot gas which is then used in a heat recovery steam generator.
When hot gases from a gas turbine enter the inlet of a heat recovery steam generator (HRSG), the gases must undergo a 10-fold reduction in velocity due to the differences in the sizes of the gas turbine outlet and the HRSG. This decrease in velocity is accomplished in a diffuser duct. Conventional ducts are asymmetrical, having a horizontally extending, flat wall, and an upwardly angled diverging wall. In typical ducts of this type, the angle of divergence of the upper wall is much more than 15.degree., i.e. the level which would allow for expansion of the gas without separation and backflow of gas inside the diffuser duct. The reasons for using a diffuser having a degree of divergence greater than 15.degree. are economic, i.e. in order to minimize costs and space requirements. Unfortunately, however, there are problems associated with gas jet separation resulting from the use of a diffuser duct having a higher degree of divergence. First, the divergence results in an increase in overall system pressure loss. Second, gas jet separation results in flow velocity profiles which are less then ideal for both duct burners and pollution control devices employed within the duct.
The conventional solution to the problem of poor flow velocity profiles has been to use flow controls within the diffuser duct. The drawback of such controls is that they are expensive and tend to increase the overall pressure loss within the system.
Conventional gas turbine combined cycle systems of the type described above include a horizontally-oriented gas turbine which sends exhaust gas into a heat recovery steam generator located next to the gas turbine in a horizontal relationship. HRSGs manufactured and used in the United States often rely upon natural circulation on the water side. Forced circulation systems also are known, and are used more commonly in Europe.